The use of rubbery polymers to toughen thermoplastic vinylic polymers is well known. This has been done with both the organic rubbers such as polybutadiene and with organosilicon polymers such as vinyl-containing organopolysiloxane gums. The toughening can be accomplished by two methods. One is by bulk or suspension polymerization in which the organosiloxane is dissolved in the monomer and the monomer is polymerized until it becomes quite viscous and the polymer is then suspended in water in relatively large droplets and the polymerization completed. Secondly, the preparation can be carried out by emulsifying the monomeric vinylic compound and polymerizing it in emulsion in the presence of the emulsified organosilicon compound.
There are certain distinct differences between these two processes. The present case is primarily concerned with the regulation of particle size of the distributed organosiloxane phase. In bulk or suspension polymerization, the particle size of the gelled organopolysiloxane distributed through the thermoplastic matrix is influenced by the reactive substituents on the silicon atoms of the organopolysiloxane. The particle size in turn effects significantly the resulting impact strength of the finished polymer. It is, of course, understood by those skilled in the art that adequate stirring is needed in bulk polymerization. On the other hand, in emulsion polymerization the particle size is determined primarily, not by the substituents on the silicon atom, but by the conditions used to emulsify the organopolysiloxane. Thus, the two processes are not equivalent and those factors which effect impact strength in bulk or suspension polymerization are not necessarily a factor in emulsion polymerization.
U.S. Pat. No. 3,923,923 teaches bulk polymerization of styrene having dissolved therein vinyl-containing siloxane. In column, 4, line 66 to column 5, line 5, patentee shows the use of mercaptoalkylsiloxane in addition to the vinylsiloxane. However, the patentee requires the presence of vinyl on the silicon and, in fact, it has been shown that the use of mercaptoalkyl compounds in conjunction with vinyl does not improve the impact strength of polystyrene and may even degrade the impact strength (see Example 7 infra). Furthermore, this patent teaches nothing about copolymers of styrene with other monomers such as acrylonitrile and in view of the failure to improve the impact strength of styrene, it would not be obvious from the reference that the use of mercaptoalkylsiloxanes alone or in combination with vinylsiloxanes would improve the impact strength of other thermoplastics.
U.S. Pat. No. 3,879,491 teaches the bulk polymerization of styrene and other vinylic monomers including some of those in the instant application, by incorporating therein vinyl-containing organopolysiloxane gums and then polymerizing the mixture with a free radical generator. However, this patent does not teach the use of mercaptoalkylsiloxanes either alone or in conjunction with vinyl and requires that vinyl be present in the organosiloxane. The basis for patentability of the instant process and materials over that disclosed in said patent is the fact that by using mercaptoalkylsiloxanes, one obtains better impact strength with the monomers claimed herein than one does with vinyl-containing siloxanes. In this connection there is an anomalous result reported in Example 3 of the U.S. Pat. No. 3,879,491 which shows a notched Izod impact strength of 2.03 foot pounds per inch of notch for a styrene-acrylonitrile copolymer modified with a vinyl-containing methylpolysiloxane. Applicants have repeated the impact test using a sample of the same plastic prepared in this example and have been unable to duplicate the 2.03 results. The results obtained by applicants on duplicate runs were 0.49 and 0.46 for the Izod impact strength and this is in line with values of 0.4 to 0.7 obtained in other experiments when they bulk polymerized styrene in the presence of vinyl-containing organosiloxane gums. By contrast, employing the process of this invention one obtains Izod impact strengths of from 1.5 to 3.5 with styrene-acrylonitrile copolymers. Consequently, the improvement in impact strengths over the process U.S. Pat. No. 3,879,491 is real and is entirely unexpected based on the teachings of this patent or a combination of this patent with the art.
Other patents such as U.S. Pat. Nos. 3,532,729 and 3,898,300 show modifying vinylic polymers by employing the emulsion polymerization technique. These references are less pertinent to the instant application for the reasons stated above with regard to the difference between emulsion and bulk polymerization. Improving impact strength of vinylic polymers using emulsion polymerization and mercaptosiloxanes is claimed in the copending application Ser. No. 679,595, filed on an even date hereinwith, of James R. Falender, Claudia M. Mettler and John C. Saam entitled "Toughened Mercaptosiloxane Modified Emulsion Polymerized Vinylic Polymers."
The object of this invention is to further improve the impact strength of bulk polymerized vinylic monomers by employing mercaptoalkylsiloxanes as the grafting site between the vinylic polymer and the organosiloxane.